fix: update reactive ref after merge in imagePreviewStore (#8479)

## Summary
Fix for COM-14110: Preview image does not display new outputs in
vue-nodes.

## Problem
The merge logic in `setOutputsByLocatorId` updated `app.nodeOutputs` but
returned early without updating the reactive `nodeOutputs.value` ref.
This caused Vue components to never receive merged output updates
because only the non-reactive `app.nodeOutputs` was being updated.

## Solution
Added `nodeOutputs.value[nodeLocatorId] = existingOutput` after the
merge loop, before the return statement.

## Testing
- Added 2 unit tests covering the merge behavior
- All 4076 existing unit tests pass
- Typechecks pass
- Lint passes

## Notes
- Related open PRs touching same files: #8143, #8366 - potential minor
conflicts possible

Fixes COM-14110

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8479-fix-update-reactive-ref-after-merge-in-imagePreviewStore-2f86d73d365081f1a145fa5a9782515f)
by [Unito](https://www.unito.io)

Co-authored-by: Amp <amp@ampcode.com>

src/stores/imagePreviewStore.test.ts

@@ -14,7 +14,9 @@ vi.mock('@/utils/litegraphUtil', () => ({
 
 vi.mock('@/scripts/app', () => ({
   app: {
-    getPreviewFormatParam: vi.fn(() => '&format=test_webp')
+    getPreviewFormatParam: vi.fn(() => '&format=test_webp'),
+    nodeOutputs: {} as Record<string, unknown>,
+    nodePreviewImages: {} as Record<string, string[]>
   }
 }))
 
@@ -29,6 +31,62 @@ const createMockOutputs = (
   images?: ExecutedWsMessage['output']['images']
 ): ExecutedWsMessage['output'] => ({ images })
 
+vi.mock('@/stores/executionStore', () => ({
+  useExecutionStore: vi.fn(() => ({
+    executionIdToNodeLocatorId: vi.fn((id: string) => id)
+  }))
+}))
+
+vi.mock('@/platform/workflow/management/stores/workflowStore', () => ({
+  useWorkflowStore: vi.fn(() => ({
+    nodeIdToNodeLocatorId: vi.fn((id: string | number) => String(id)),
+    nodeToNodeLocatorId: vi.fn((node: { id: number }) => String(node.id))
+  }))
+}))
+
+describe('imagePreviewStore setNodeOutputsByExecutionId with merge', () => {
+  beforeEach(() => {
+    setActivePinia(createTestingPinia({ stubActions: false }))
+    vi.clearAllMocks()
+    app.nodeOutputs = {}
+    app.nodePreviewImages = {}
+  })
+
+  it('should update reactive nodeOutputs.value when merging outputs', () => {
+    const store = useNodeOutputStore()
+    const executionId = '1'
+
+    const initialOutput = createMockOutputs([{ filename: 'a.png' }])
+    store.setNodeOutputsByExecutionId(executionId, initialOutput)
+
+    expect(app.nodeOutputs[executionId]?.images).toHaveLength(1)
+    expect(store.nodeOutputs[executionId]?.images).toHaveLength(1)
+
+    const newOutput = createMockOutputs([{ filename: 'b.png' }])
+    store.setNodeOutputsByExecutionId(executionId, newOutput, { merge: true })
+
+    expect(app.nodeOutputs[executionId]?.images).toHaveLength(2)
+    expect(store.nodeOutputs[executionId]?.images).toHaveLength(2)
+  })
+
+  it('should assign to reactive ref after merge for Vue reactivity', () => {
+    const store = useNodeOutputStore()
+    const executionId = '1'
+
+    const initialOutput = createMockOutputs([{ filename: 'a.png' }])
+    store.setNodeOutputsByExecutionId(executionId, initialOutput)
+
+    const newOutput = createMockOutputs([{ filename: 'b.png' }])
+
+    store.setNodeOutputsByExecutionId(executionId, newOutput, { merge: true })
+
+    expect(store.nodeOutputs[executionId]).toStrictEqual(
+      app.nodeOutputs[executionId]
+    )
+    expect(store.nodeOutputs[executionId]?.images).toHaveLength(2)
+  })
+})
+
 describe('imagePreviewStore getPreviewParam', () => {
   beforeEach(() => {
     setActivePinia(createTestingPinia({ stubActions: false }))

src/stores/imagePreviewStore.ts

@@ -148,6 +148,7 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
             existingOutput[k] = newValue
           }
         }
+        nodeOutputs.value[nodeLocatorId] = existingOutput
         return
       }
     }